The present invention generally relates to electrical system testing. In particular, the invention relates to systems and methods for performing time domain measurements, such as differential impedance measurements, of electrical systems.
Several types of time domain measurements are used to determine characteristics of electrical systems. For instance, time domain reflectometry (TDR) measurements involve propagating a signal down a transmission line to a device under test (DUT) and then measuring the reflections from the DUT. The reflections then are evaluated to determine characteristics, such as discontinuities and/or impedances, of the DUT.
Time domain transmission (TDT) measurements also are used. TDT measurements are made by propagating a signal through a DUT. Parameters typically measured by TDT include gain and propagation delay. TDT measurements also can characterize crosstalk between traces.
Typically, a digital communications analyzer (DCA) is used to perform TDR/TDT measurements. A DCA is connected to a DUT via transmission media, such as cables, connectors and/or probes, that propagate signals to and/or from the DUT. A step generator of the DCA typically is used to generate test signals for providing to the DUT and an oscilloscope of the DCA is used to view the DUT response.
Unfortunately, a DCA can introduce error in TDR/TDT measurements. By way of example, the transmission media between the step generator, the DUT and the oscilloscope can affect measurement results. In particular, impedance mismatches and imperfect connectors add reflections to the actual signal being measured. These can distort the signal and make it difficult to determine which reflections are from the DUT and which are from other sources. Additionally, cable losses are frequency dependent and can cause the risetime of edges to xe2x80x9cdroopxe2x80x9d as they approach their final values.
Oscilloscopes also can introduce errors into measurements. For instance, the finite bandwidth of an oscilloscope translates to limited risetime. Edges with risetimes less than the minimum risetime of the oscilloscope are measured slower than the actual risetimes. Therefore, when measuring a DUT response to an edge, the limited risetime of an oscilloscope may distort or hide some of the DUT response.
The shape of the step stimulus also can be important for accurate TDR/TDT measurements. This is because the DUT responds not only to the step, but also to aberrations on the step, such as overshoot and non-flatness. Therefore, if the step overshoot is substantial, the DUT response can be more difficult to interpret.
As is known, waveform subtraction has been used to reduce the effects of measurement errors. Waveform subtraction involves obtaining a reference waveform from a known xe2x80x9cgoodxe2x80x9d reference device, and then subtracting the reference waveform from a waveform measured from the DUT. The result shows how the DUT response differs from the reference response. This technique removes error terms common to both the reference and the DUT waveforms, such as trigger coupling, channel crosstalk and reflections from the cables and connectors.
Waveform subtraction, however, has several shortcomings. First, it requires that a known xe2x80x9cgoodxe2x80x9d reference DUT exists. In some cases a xe2x80x9cgoodxe2x80x9d DUT may not exist or may not be readily available. Second, since the waveform that results from the subtraction process is a description of how the DUT response differs from the reference response, the errors introduced by the test system are difficult to isolate from the actual DUT response.
Based on the foregoing, it should be appreciated that there is a need for improved systems and methods that address the aforementioned and/or other perceived shortcomings of the prior art.
Systems and methods of the present invention enable time domain reflectometry (TDR) and/or time domain transmission (TDT) measurements to be more accurately produced compared with prior art TDR/TDT techniques. Normalization provided by these systems and methods can be used in TDR/TDT measurements to remove oscilloscope response, step aberrations, cable losses and/or reflections so that the response measured is that of the device under test (DUT). In addition, normalization can be used to predict how the DUT would respond to an ideal step of an arbitrary risetime. In some embodiments, normalization can be implemented by a time domain measuring system, which may not require the use of external controllers, multiple step generators and/or risetime converters for performing testing.
In this regard, an embodiment of a system of the invention for performing time domain measurements of a DUT includes a normalization system. The normalization system receives information corresponding to a model of a test system that is used for providing differential input signals to a DUT. The normalization system also receives information corresponding to first and second differential input signals provided to the DUT, as well as information corresponding to first and second reflected waveforms corresponding to the DUT response to the first and second differential input signals. The normalization system then computes first and second normalized waveforms using at least a first inverse transfer function of the test system. Note, the first and second normalized waveforms include fewer test system error components than the first and second reflected waveforms, respectively.
An embodiment of a method of the invention for performing time domain measurements of a DUT includes: providing a model of a test system used for providing differential input signals to a DUT; receiving information corresponding to first and second differential input signals provided to the DUT; receiving information corresponding to first and second reflected waveforms corresponding to the DUT response to the first and second differential input signals; and computing first and second normalized waveforms using at least a first inverse transfer function of the test system, the first and second normalized waveforms including fewer test system error components than the first and second reflected waveforms, respectively.
Computer-readable media also are provided. In this regard, an embodiment of a computer-readable medium of the invention for performing time domain measurements of a DUT includes: logic configured to provide a model of a test system used for providing differential input signals to a DUT; logic configured to receive information corresponding to first and second differential input signals provided to the DUT; logic configured to receive information corresponding to first and second reflected waveforms corresponding to the DUT response to the first and second differential input signals; and logic configured to compute first and second normalized waveforms using at least a first inverse transfer function of the test system, the first and second normalized waveforms including fewer test system error components than the first and second reflected waveforms, respectively.
Clearly, some embodiments of the invention may not exhibit one or more of the advantages and/or properties set forth above. Additionally, other systems, methods, features and/or advantages of the present invention will be or may become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features and/or advantages be included within this description, be within the scope of the present invention, and be protected by the accompanying claims.